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Swelling Australian cities harbour ever bigger spiders

As if Australia’s spiders weren’t big and scary enough, it turns out denser, busier cities are allowing some of them to grow even bigger. The same thing could be happening the world over.

Most Australians will have seen the large but mostly harmless golden orb-weavers that sometimes congregate in people’s gardens. Elizabeth Lowe of the University of Sydney was surprised at just how large some were growing, and began investigating what could be behind this.

Lowe and colleagues found one species of this genus, Nephila plumipes, is gaining weight the more built-up Sydney gets. Examining more than 200 specimens around Sydney, they found that the more concrete there was, the further they were from bushland and the less leaf litter there was, the bigger the spiders tended to be. Lowe says the spiders in the bushland of Brisbane Water National Park, north of Sydney, had an average mass of 0.5 grams. But those in an inner-city park near Bondi Beach averaged 1.6 grams.

“It’s probably because of the urban heat-island effect and prey availability,” says Lowe. “Most invertebrates will grow to larger sizes if they are warmer. They are very sensitive to temperatures.”

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Urbanisation probably benefits these spiders in several ways, she says. The insects the spiders prey on tend to do very well in small fragments of bushland like urban parks, and even better when there is lighting at night. “The spiders have more to prey on and can put more energy into growing bigger.”

Wealth effect

Surprisingly, the researchers also found that wealthier areas tended to have larger spiders. Lowe isn’t sure why. It might be that wealthier areas tend to have more parks or more hard surfaces like concrete that heat up readily.

Lowe says this is probably also happening with other spiders in Australia like huntsmen and redbacks, and is almost certainly happening elsewhere in the world. What’s more, global warming will encourage spiders to grow bigger – although very hot weather will kill them, she says. “During the hot summer last year, most of these spiders died out in Sydney.”

Michael Kasumovic from the University of New South Wales in Sydney said he is surprised by the results and expected the opposite would be true. “If it is an increase in heat, what that usually means is it increases development rates. Metabolic functions are going faster and they mature earlier – at a smaller size,” he says.

Lowe notes the relationship between heat and body size is complicated. The heat could allow the spiders to hatch earlier in the year, giving them a longer growing season.

In any case, she says healthy spider populations in cities should be celebrated&colon; they are mostly harmless to people, they eat pests and they provide food for birds. “I’m all about encouraging spiders in urban areas,” she says.